Li metal anodes have attracted attention due to the possibility of developing high energy density batteries. Unfortunately, rechargeable lithium batteries employing lithium metal anodes have not been commercialized, mainly due to issues of safety and cycle life properties. These difficulties are due mainly to the Li metal anode. In particular, the Li metal is highly reactive with most organic compounds, which limits the selection of an electrolyte. Also, during charging and discharging, lithium dendrites gradually grow out from the lithium anode surface and into the electrolyte. Left unperturbed, the dendrites can eventually contact the cathode and short the cell. Various approaches have been suggested to address the reactivity of the Li metal anode in liquid electrolytes, but without success.
It has been proposed to provide a protective layer on the electrolyte-facing side of the lithium metal layer. Such protective layers should conduct lithium ions, while simultaneously preventing electrolyte from contacting the anode metal; such approaches are discussed, for example, in U.S. Patent Application Publication No. 2008/0057386.
One previously-suggested approach to reducing dendrite growth and increasing cycle life has been to use a single ion conductor coating such as glassy or amorphous single metal oxides. In other approaches, an alloy of a metal with lithium has been used. Other inorganic additives have been deposited using vacuum deposition. In other approaches, combinations of inorganic, e.g., single ion conductor, and organic polymer layers have been used as protective coatings. More recently, compositions employing organic polymer have been employed. However, the polymer coating layers have not demonstrated sufficient mechanical strength and are easily shattered or damaged by swelling due to solvation with the electrolyte.
An effective and efficient way to increase the cycle life and increase the safety of Li metal high density batteries is still needed.